A. Technical Field
The field of the invention is best described as entailing lithographic delineation by selective removal of portions of continuous layers. Of particular concern is the fabrication of integrated circuits, particularly silicon integrated circuits. Fabrication of such circuits involves selective removal of elemental silicon, silicon-containing compounds, as well as associated materials, such as, aluminum conductor.
B. History
Large Scale Integrated circuitry and particularly that involving silicon (Silicon Integrated Circuits) represents a high level of sophistication. Commercial circuitry is based on design features of three or four micrometers. Circuit chips a fraction of an inch in size may contain fifty thousand or more elements. Experimentally designed and constructed circuits are based on still smaller feature size. The emerging level known by some as Very Large Scale Integrated circuitry contemplates chips with hundreds of thousands of elements or even a million elements.
Many types of processing are involved in the construction of LSI devices. SIC's, for example, entail a wide array of deposition and other material addition procedures. Regardless of the type of circuitry, all manufacture involves etching--first in delineation of masking layers and, finally, in delineation of device material layers. Selectivity, whether by removal or addition, is generally based on extremely precise delineation brought about by etch-removal of selected regions of continuous layers. These etch procedures (including those specific to resist removal--sometimes called resist development) have necessarily kept pace with device miniaturization and must be considered the critical determinant of both presently available and prospective levels of integration.
Wet etching in which selected removal results from differential solubility or selected baring of material to be removed relies on a variety of agents--both organic and inorganic. Wet etching continues to be a significant factor. At the resist pattern delineation level, wet etching, more commonly called wet development, is prevalent.
To an increasing extent, wet etching is yielding to dry etching. Advantages, of increasing significance for further device miniaturization include reduction in amount of residue. Inherent lithographic advantages include a high degree of resolution, as well as some degree of control over etch profile.
Many recent developments involve systems specific to the variety of materials to be removed. A significant category of systems are based on etchant species which directly or indirectly involve charged particles or other unstable particles produced through electrical discharge across a vapor medium. Such plasma assisted processes take a variety of forms depending on electrical input (power, frequency, etc.) and depending on apparatus design (electrode size, spacing). Plasma-assisted etching is often carried out within or adjacent to the luminescent region of the plasma. See, C. M. Melliar-Smith and C. J. Mogab in Thin Film Processes J. L. Vossen and W. Kern, eds. Chap. V-2 Academic Press, New York (1978). A variant involves "downstream etching" in which plasma produced species contact material to be removed only at a position downstream of the active discharge.
LSI devices undergoing fabrication involve delineation of layers which may be a micrometer or less in thickness. Spacings between regions to be removed may be a very small number of micrometers. Development of plasma chemistry with a high degree of discrimination as between material to be removed and retained, as well as systems which permit anisotropic or other desired profiles have been required and are generally acceptable. More and more, feasibility of finer design rules, commensurate with reasonable yield, have depended upon other considerations.
Discrimination and profile control often require monitoring. For example, etching should be stopped when underlying material has been bared. Failure to reach this "endpoint" may destroy device function; while exceeding it may result in lateral attack on etch walls to result in undercutting or "bottoming". Monitoring, and particularly endpoint detection is a significant part of the technology.
Endpoint detection may take the form of detection of an impedance change. Impedance, as measured across the discharge region, is altered by presence in kind and amount of etchant species as well as by fragments produced by wanted or unwanted material removal. An alternative endpoint detection approach is based upon emission within the plasma. Such measurement may be based on emission from an excited plasma species--e.g., on emission from a plasma etched species which changes amplitude based on the degree of consumption during etching. An example is monitoring based on the 703.7 nm emission line of impact-excited atomic fluorine. Both procedures are used to advantage--both are limited: (a) both involve small differences in relatively large quantities; (b) both entail averaging and consequently may not indicate certain types of product failure.